The effect of electrostatic charging has been recorded throughout history. For example, the ancient Greeks noted that when amber was rubbed with cat's fur it started to attract small pieces of almost any light weighted material. Since then we have learned much about this phenomenon, and today electrostatic charging is used in a wide variety of products ranging from laser printers to air cleaners. However, we have also learned that a large build-up of electrostatic charges followed by an uncontrolled discharge, also known as an electrostatic discharge (ESD) event, may severely damage an electronic device.
The generation of an ESD event is mainly due to either triboelectrification or induction charging. Triboelectrification, or tribocharging, occurs when two materials are brought into contact and then separated, causing a separation of electric charges. Typical examples of tribocharging include walking on a rug with insulated footwear, descending from a car, or rubbing a balloon in the hair. In all these cases, the friction between two materials creates a difference of electrical potential which might lead to an ESD event when the charges object comes into contact with a grounded object.
Induction charging occurs when an electrically charged object is placed in the vicinity of a conductive object which is isolated from ground. The presence of the charged object creates an electrostatic field that causes electrical charges on the surface of the other object to redistribute into regions of excess positive and negative charges. An ESD event may then occur when the object comes into contact with a conductive path. An example of this is for instance when charged regions on the surfaces of a plastic bag induce potential on a nearby electronic component, and when the component is touched with a grounded metallic tool an ESD event occurs.
As mentioned above electronic devices may be damaged by and ESD event. This is due to the fact that modern-day semiconductor devices are very sensitive to ESD, and when subjected to an ESD event the devices may suffer direct, indirect and latent damage. The direct damage results from physical destruction, or degradation, of an electronic part in the device, which makes it unusable. This failure is classified as a hard failure since it is irreversible. An indirect damage is the result of when for instance a device changes its state due to conducted or radiated electromagnetic interference (EMI) sourced by the discharge. This failure is classified as soft failure. A latent damage is when a discharge affects the device which does not show any apparent damage but with time causes a hard failure to occur. Because of the increase in use of semiconductor devices in every aspect of life, it has become an important issue to protect electronic devices as much as possible from damage or malfunction due to ESD.
The most common ways of minimizing the effects of an ESD event on an electronic device is to either add protective circuitry, apply shielding to ESD sensitive circuitry in the device, or to construct an elaborate ground system which can dissipate the high transient currents generated during an ESD event. Although these precautions may protect the device from suffering hard failures, soft failures resulting in erroneous behaviour or hang-ups of the device may still occur due to the EMI generated by discharges.